Add usage

Author: shihanng

README.md

@@ -9,3 +9,17 @@ iruby-docker is a community maintained Jupyter Docker Stack image
 with [iruby](https://github.com/SciRuby/iruby) installed.
 This image is built based on the [Jupyter Docker Stacks's cookiecutter](https://jupyter-docker-stacks.readthedocs.io/en/latest/contributing/stacks.html),
 the image is available on [Docker Hub](https://hub.docker.com/r/shihanng/iruby-docker).
+
+## Usage
+
+```
+$ docker run -it --name iruby-notebook --rm -p 8888:8888 -v `pwd`:/home/jovyan/work shihanng/iruby-docker:latest
+...
+
+    To access the notebook, open this file in a browser:
+        file:///home/jovyan/.local/share/jupyter/runtime/nbserver-17-open.html
+    Or copy and paste one of these URLs:
+        http://(xxxxxxxxxxxx or 127.0.0.1):8888/?token=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+```
+
+Based on the URL above, visit `http://127.0.0.1:8888/?token=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx`.